Method for preparing a hydro(chloro)fluoroalkane and catalyst

ABSTRACT

Process for the preparation of a hydro(chloro)fluoroalkane according to which a halogenated precursor of the hydro(chloro)fluoroalkane is reacted with hydrogen fluoride in the presence of a catalyst comprising chromium (Cr) and at least one other metal selected from the group consisting of aluminium, barium, bismuth, calcium, cerium, copper, iron, magnesium, strontium, vanadium and zirconium.

This application is the national stage of PCT/EP01/0498 filed Apr. 9,2001 and published as WO01/77048 on Oct. 18, 2001.

The present invention relates to a process for the preparation of ahydro(chloro)fluoroalkane and a catalyst.

When a halogenated precursor of a hydro(chloro)fluoroalkane is reactedin the presence of hydrogen fluoride under fluorination conditions, itis difficult to selectively obtain a desired hydro(chloro)fluoroalkane.The formation of other reaction products, which often cannot be recycledor used, such as undesired isomers or dismutation products, reduces theproductive output of hydro(chloro)fluoroalkane.

It is, for example, known from JP-A-02/178237 to carry out ahydrofluorination of perchloroethylene with hydrogen fluoride in thepresence of a catalyst comprising chromium, magnesium and aluminiumoxides. This process has an unsatisfactory productive output ofpentafluoroethane (HFC-125). The process is not more satisfactory withregard to the selectivity for 1,1,1-trifluoro-2,2-dichloroethane(HCFC-123).

It was therefore desirable to find a process which would make possibleaccess in a controlled and selective manner to a specifichydro(chloro)-fluoroalkane.

The invention consequently relates to a process for the preparation of ahydro(chloro)fluoroalkane according to which a halogenated precursor ofthe hydro(chloro)fluoroalkane is reacted with hydrogen fluoride in thepresence of a catalyst comprising chromium (Cr) and at least one othermetal (M) selected from aluminium, barium, bismuth, calcium, cerium,copper, iron, magnesium, strontium, vanadium and zirconium, in whichprocess the catalyst is depleted in ammonium ions.

It has been found, surprisingly, that the process according to theinvention makes possible access with high selectivity to a specifichydro(chloro)-fluoroalkane with high overall selectivity forhydro(chloro)fluoroalkane and for hydrochlorofluorinated precursors ofthe said hydro(chloro)fluoroalkane.

In the process according to the invention, the catalyst typicallyincludes at most 1% by weight of ammonium ions. It preferably exhibits acontent of ammonium ions of at most 0.5% by weight. The content ofammonium ions in the catalyst is preferably at most 0.2% by weight.Excellent results are obtained with a catalyst for which the content ofammonium ions is at most 0.1% by weight. Particularly advantageousresults are obtained with a catalyst for which the content of ammoniumions is at most 0.05% by weight.

The catalyst can be a bulk or supported catalyst. A bulk catalyst ispreferred.

The M/Cr atomic ratio of the metal M to the chromium in the catalyst isgenerally at least 0.01. The atomic ratio is preferably at least 0.05,advantageously at least 0.1. The atomic ratio is generally at most 100.It is most often at most 20. It is preferably at most 10.

The catalyst generally exhibits a specific surface determined accordingto the BET method with nitrogen, of at least 15 m²/g, preferably of atleast 25 m²/g. The specific surface is generally at most 200 m²/g. It ispreferably at most 100 m²/g.

In the process according to the invention, the catalyst is preferablyobtained by fluorination of a mixed oxide of chromium and of metal M.The fluorination is preferably carried out with hydrogen fluoride,optionally diluted with an inert gas, such as nitrogen or helium. Theduration of the fluorination is generally from 1 to 100 h. Thefluorination temperature is generally between 150 and 400° C. It ispreferably at most 350° C. The fluorination can be carried out, forexample, immediately before the reaction of the halogenated precursorwith hydrogen fluoride, preferably in the reactor used for the latterreaction.

The preparation of the mixed oxide preferably comprises a stage ofcoprecipitation by reaction of an aqueous solution of soluble metal andchromium salts, which is reacted an aqueous ammonia solution.

The preparation of the mixed oxide can advantageously comprise one ormore drying or calcination stages. The temperature of the calcination isgenerally from 150° C. to 400° C. The calcination temperature ispreferably at most 350° C. A calcination temperature of at most 340° C.is more particularly preferred. The calcination is often carried out ata temperature of at least 200° C.

After calcination, the mixed oxide generally exhibits a specificsurface, determined according to the BET method with nitrogen, of atleast 150 m²/g, preferably of at least 180 m²/g. The specific surface ofthe mixed oxide is generally at most 450 m²/g.

The preparation of the catalyst generally comprises a treatment intendedto reduce, preferably to essentially completely remove, the content ofammonium ions in the catalyst. This treatment can, for example, be atleast one washing, preferably with water, or a heat treatment or acombination of these treatments. The effectiveness of the treatmentintended to reduce the content of ammonium ions is generally confirmedby methods known per se. For example, Nessler's reagent can be used toanalyse the content of ammonium ions. This treatment is preferablyapplied to the mixed oxide precursor of the catalyst, before subjectingit to the fluorination stage.

In a particularly preferred alternative form, the preparation of thecatalyst comprises the following stages:

-   (a) a preparation of a mixed oxide of metal M and of chromium by    coprecipitation from an aqueous solution of soluble metal and    chromium salts, which is reacted with an aqueous ammonia solution;-   (b) a washing of the mixed oxide, intended to reduce, preferably to    essentially completely remove, the content of ammonium ions in the    catalyst;-   (c) a calcination of the washed mixed oxide;-   (d) a fluorination treatment of the calcined mixed oxide.

The catalyst and the mixed oxide are generally essentially amorphous.They are preferably completely amorphous.

In the process according to the invention, the reaction between thehydrogen fluoride and the chlorinated precursor is usually carried outat a temperature of 150 to 450° C. The reaction is preferably carriedout in the gas phase. The pressure of the reaction is usually from 0.5to 30 bar. The molar ratio of the hydrogen fluoride to the halogenprecursor is usually from 1 to 100. The residence time is usually from 1to 1000 s.

Halogenated precursors which can be used in the process according to theinvention are known. The halogenated precursor is preferably achlorinated precursor. Mention may be made, by way of example, ofdichloromethane, trichloromethane, trichloroethylene, perchloroethyleneand pentachloroethane.

In an alternative form, the process according to the invention appliesto the synthesis of a hydrochlorofluoroalkane. In this alternative form,the metal M is advantageously selected from barium, bismuth, copper,iron, magnesium and strontium. The metal is preferably selected frombarium, bismuth, magnesium and strontium. This alternative form of theprocess according to the invention has proved to be advantageous forpreparing chlorofluoromethane, chlorodifluoromethane,2-chloro-1,1,1-trifluoroethane or 2,2-dichloro-1,1,1-trifluoroethane.

This alternative form has proved to be particularly advantageous forpreparing 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123). In this case,the halogenated precursor advantageously comprises at least onechlorinated organic compound selected from perchloroethylene,1,1,2-trichloro-2-fluoroethylene, 1,1,2-trichloro-2,2-difluoroethane and1,1,2,2-tetrachloro-2-fluoroethane. The chlorinated organic compound ispreferably selected from 1,1,2-trichloro-2,2-difluoroethane andperchloroethylene.

In another alternative form, the process according to the inventionapplies to the synthesis of a hydrofluoroalkane. In this alternativeform, the metal is advantageously selected from aluminum, calcium,cerium, vanadium and zirconium. Zirconium and aluminum are preferred.Aluminium is particularly preferred.

In order to reduce the cost of the catalyst in the process according tothe invention, it is advantageous to use a catalyst with a high contentof aluminium with respect to the more expensive chromium. It has beenfound that, in the process according to the invention,hydrofluoroalkanes can be synthesized in a particularly selective andeconomic manner when use is made of a catalyst comprising chromium andaluminium in an Al/Cr atomic ratio of at least 0.5. This ratio ispreferably at least 1. In a particularly preferred way, this ratio is atleast 2. The Al/Cr atomic ratio is advantageously at most 20. This ratiois preferably at most 10. In a particularly preferred way, it is at most5.

This second alternative form has proved to be advantageous for preparingdifluoromethane, 1,1,2-tetrafluoroethane or pentafluoroethane.

This alternative form has proved to be particularly advantageous forpreparing pentafluoroethane. In this case, the halogenated precursor isadvantageously perchloroethylene or 2,2-dichloro-1,1,1-trifluoroethane.The pressure in this case is advantageously at most 15 bar, preferablyless than 10 bar. The pressure is advantageously at least 1 bar. A lowerpressure makes possible an additional increase in the selectivity forpentafluoroethane whereas a higher pressure makes it possible toincrease the overall productive output of the process.

The invention also relates to the mixed oxide and to the catalyst whichare described above.

The examples given below are intended to illustrate the inventionwithout, however, limiting it. In the examples, the degree of conversionof the halogenated precursor is the ratio, expressed as per cent, of, onthe one hand, the amount employed minus the unconverted amount to, onthe other had, the amount employed; the selectivity forhydro(chloro)fluoroalkane is the ratio, expressed as per cent, of theamount of hydro(chloro)fluoroalkane formed to the amount which wouldhave been formed if all the halogenated precursor converted hadgenerated hydro(chloro)fluoroalkane; the overall selectivity is the sumof the selectivity for the desired hydro(chloro)fluoroalkane of all therecoverable intermediates; the yield of hydro(chloro)fluoroalkane is theproduct of the degree of conversion and the selectivity for thishydro(chloro)fluoroalkane.

EXAMPLES 1–7

1.61 of an aqueous solution comprising 0.56 mol/l of nitrates ofchromium and of other metal M and exhibiting the desired atomic ratio ofthe chromium to the metal M were prepared. 0.7 l of aqueous ammoniasolution, exhibiting a concentration of NH₄OH of 4 mol/l, was added tothe solution with stirring at ambient temperature. The precipitate wascentrifuged. The cake was washed several times with water atapproximately 65° C. until at least 80% of the initial content ofammonium ions in the cake, confirmed using Nessler's reagent, had beenremoved. The washed cake was dried for 2 days at 105° C. Theagglomerates obtained after drying were milled to produce grains with asize of less than 5 mm. The grains were subjected to calcination for atotal period of 69 h while flushing with nitrogen. The temperature wasmaintained first at 215° C. and then at 330° C. The specific surface(SS) of the mixed oxide obtained is shown in Table 1. It was determinedaccording to the BET method with nitrogen, measured on a Carlo ErbaSorptomatic® 1990 device after degassing the samples at ambienttemperature for 12 h under a vacuum of 10⁻⁵ torr. The content ofammonium ions in the mixed oxide is also shown in Table 1.

The mixed oxide was introduced into a tubular reactor made of HastelloyC. A fluorination treatment was carried out with hydrogen fluoride gas(10 g/h per 100 cm³ of mixed oxide) mixed with nitrogen for 8 h at atemperature of 200 to 350° C. The specific surface (SS) of the catalystswhich are obtained is shown in Table 1.

10 cm³ of the catalyst and a hydrogen fluoride/ perchloroethylene (PER)mixture in a molar ratio of 10 mol/mol were introduced into a tubularreactor with an internal diameter of 15 mm. The reaction pressure wasmaintained at 1 bar and the temperature at 350° C. The residence timewas 12.5 seconds.

The results are collated in Table 1 below. It emerges therefrom that theprocess according to the invention makes it possible to obtain a highyield of HFC-125. The process gives a high overall selectivity at highdegrees of conversion.

TABLE 1 Over- De-gree all SS Mixed SS of con- selec- HFC- M/Cr mixedoxide cata- ver- tiv- 125 atomic oxide NH₄ ⁺ con- lyst sion ity yield NoMetal ratio (m²/g) tent (g/kg) (m²g) (%) (%) (%) 1 Al 10/90 299 0.05 15597.5 89.8 61.0 2 Al 50/50 351 0.003 78 81.1 92.4 36.7 3 Al 70/30 343<0.002 68 89.3 94.4 48.0 4 Al 90/10 351 0.004 54 79.2 94.0 35.1 5 Zr10/90 238 <0.002 77 98.0 92.8 649 6 V* 10/90 254 0.007 106 95.0 91.361.1 7 Ca 10/10 291 <0.002 26 76.0 88.7 42.6 *Vanadium (III) chloridewas used in the preparation

1. A process for the preparation of a hydro(chloro)fluoroalkaneaccording to which a halogenated precursor of thehydro(chloro)fluoroalkane is reacted with hydrogen fluoride in thepresence of a catalyst comprising chromium (Cr) and at least one othermetal (M) selected from aluminium, barium, bismuth, calcium, cerium,copper, iron, magnesium, strontium, vanadium and zirconium, in whichprocess the content of ammonium ions NH₄ ⁺ in the catalyst is presentand is at most 1% by weight.
 2. A process according to claim 1, in whichthe content of ammonium ions in the catalyst is present and is in anamount at most 0.2% by weight.
 3. A catalyst comprising chromium (Cr)and at least one other metal (M) selected from aluminium, barium,bismuth, calcium, cerium, copper, iron, magnesium, strontium, vanadiumand zirconium, in which catalyst the content of ammonium ions NH₄ ⁺ ispresent and is in an amount at most 1% by weight.
 4. The catalystaccording to claim 3, in which the content of ammonium ions NH₄ ⁺ is atmost 0.2% by weight.
 5. A mixed oxide comprising chromium (Cr) and atleast one other metal selected from aluminium, barium, bismuth, calcium,cerium, copper, iron, magnesium, strontium, vanadium and zirconium, inwhich mixed oxide the content of ammonium ions NH₄ ⁺ present and is inan amount at most 1% by weight.
 6. A mixed oxide according to claim 5,in which the content of ammonium ions NH₄ ⁺ is at most 0.2% by weight.7. The process according to claim 1, wherein the catalyst exhibits aspecific surface, determined according to the BET method, of at least 15m²/g.
 8. The process according to claim 1, wherein the catalyst wasobtained by fluorination of a mixed oxide of chromium and of metal M. 9.The process according to claim 8, wherein the mixed oxide was obtainedby coprecipitation by reaction of an aqueous solution of soluble metaland chromium salts, which is reacted with an aqueous ammonia solution.10. The process according to claim 8, wherein the mixed oxide exhibits,before fluorination, a specific surface, determined according to the BETmethod, of at least 150 m²/g.
 11. The process according to claim 1,applied to the synthesis of a hydrofluoroalkane andhydrochlorofluoralkane.
 12. The process according to claim 11, whereinthe metal is selected from the group consisting of aluminium, calcium,cerium, vanadium and zirconium.
 13. The process according to claim 12,wherein the metal is aluminium.
 14. The process according to claim 13,wherein the Al/Cr atomic ratio is from 0.5 to
 20. 15. The processaccording to claim 11, wherein the hydrofluoroalkane obtained ispentafluoroethane.
 16. The process according to claim 1, in which themetal is selected from barium, bismuth, copper, iron, magnesium andstrontium.